The present disclosure relates to exhaust gas-driven turbochargers having variable turbine nozzles, and relates in particular to variable nozzles having the capability of bypassing some of the exhaust gas around the turbine wheel.
Regulation of the exhaust gas flow through the turbine of an exhaust gas-driven turbocharger provides known operational advantages in terms of improved ability to control the amount of boost delivered by the turbocharger to the associated internal combustion engine. The regulation of exhaust gas flow is accomplished by incorporating variable geometry into the nozzle that leads into the turbine wheel. By varying the size of the nozzle flow area, the flow into the turbine wheel can be regulated, thereby regulating the overall boost provided by the turbocharger's compressor.
Variable-geometry nozzles for turbochargers generally fall into two main categories: variable-vane nozzles, and sliding-piston nozzles. Vanes are often included in the turbine nozzle for directing the exhaust gas into the turbine in an advantageous direction. Typically a row of circumferentially spaced vanes extend axially across the nozzle. Exhaust gas from a generally annular chamber surrounding the turbine wheel flows generally radially inwardly through passages between the vanes, and the vanes turn the flow to direct the flow in a desired direction into the turbine wheel. In most variable-vane nozzles, the vanes are rotatable about their axes to vary the angle at which the vanes are set, thereby varying the flow area of the passages between the vanes.
In the sliding-piston type of nozzle, the nozzle may also include vanes, but the vanes are fixed in position. Variation of the nozzle flow area is accomplished by an axially sliding piston that slides in a bore in the turbine housing. The piston is tubular and is located just radially inwardly of the nozzle. Axial movement of the piston is effective to vary the axial extent of the nozzle leading into the turbine wheel, thus varying the “throat area” at the turbine wheel inlet. When vanes are included in the nozzle, the piston can slide adjacent to radially inner (i.e., trailing) edges of the vanes; alternatively, the piston and vanes can overlap in the radial direction and the piston can include slots for receiving at least a portion of the vanes as the piston is slid axially to adjust the nozzle.
There are times during the operation of a turbocharger when it is desired to pass as much flow through the turbine as possible. For example, it may be desirable to minimize the backpressure felt by the engine during certain operating conditions, and this is accomplished by reducing the flow restriction downstream of the engine as much as possible. In a sliding piston-type variable turbine nozzle, the downstream flow restriction is reduced by fully opening the piston to maximize the throat area at the turbine wheel inlet. However, in some cases, even fully opening the piston may not allow as much flow to pass as may be desired. Accordingly, some piston-type variable nozzles are configured to have a bypass passage that is opened when the piston is slid to a fully open position. The bypass passage extends between the generally annular chamber of the turbine housing and the bore in the turbine housing, such that exhaust gas flows from the chamber to the bore, bypassing the turbine wheel. Some turbines with variable-vane nozzles similarly include a bypass valve for bypassing the turbine wheel.
Both the variable-vane and sliding piston types of variable nozzles have advantages and disadvantages. For example, variable-vane nozzles having rotatable vanes generally have good aerodynamic performance, but are mechanically complex because of the substantial number of moving parts. Sliding piston-type variable nozzles are mechanically much simpler, having far fewer moving parts, but generally are not as good aerodynamically as variable-vane nozzles.
There is a third category of variable nozzle, represented for example by international patent application publication WO 2004/074643 to Lombard et al., in which the variable nozzle includes vanes each of which is made up of a fixed leading-edge portion (a fixed vane) and a movable trailing-edge portion (a movable vane). The movable vanes are supported on a rotor that is rotated about its axis to vary the positions of the movable vanes relative to the fixed vanes. This design offers mechanical simplicity combined with some of the advantages of vane-type nozzles.